ARTICLE pubs.acs.org/JPCC
Impact of the 3d Electronic States of Cobalt and Manganese Phthalocyanines on the Electronic Structure at the Interface to Ag(111) F. Petraki,*,† H. Peisert,† F. Latteyer,† U. Ayg€ul,† A. Vollmer,‡ and T. Chasse† † ‡
University of Tuebingen, IPTC, Auf der Morgenstelle 18, 72076 Tuebingen, Germany Helmholtz Centre Berlin for Materials and Energy, Electron Storage Ring BESSY II, Albert-Einstein-Str. 15, 12489 Berlin, Germany ABSTRACT: X-ray absorption (XAS) and photoemission (XPS) spectroscopy revealed a strong interaction at the interface between transition-metal Cobalt phthalocyanine (CoPc) and single-crystalline Ag(111) substrate. According the Co L-edge absorption spectra of ultrathin CoPc films, charge transfer between the metal 3d electrons and the underlying metallic substrate occurs. Less intense but comparable effects are also observed at the MnPc/Ag(111) interface for partially filled d levels of Mn. The nature of interacting orbitals is also discussed by means of resonant photoemission spectroscopy (ResPES) as well as in comparison with the CoPc/Au(100) interface studied previously.
’ INTRODUCTION Metal phthalocyanines have been the subject of much interest owing to their unique electronic and optical properties that can be technically exploited.1 7 For example, it has been demonstrated recently that in particular transition-metal phthalocyanines (TMPcs) possess interesting magnetic properties due to the presence of the partially empty d levels of the central transitionmetal atom,8 12 and their application to spintronic devices is under investigation.13 16 Therefore, detailed experimental studies of the interfaces between TMPcs and metallic substrates can give valuable insight into the understanding of the processes that take place at device interfaces. In particular, with respect to magnetic devices, the spin situation of the central metal atom of the TMPc may change distinctly at the interfaces. Because TMPcs are small molecules that can be easily evaporated under controlled vacuum conditions, they are well-suited as representative model systems for the investigation of the fundamental properties (electronic, optical, and magnetic) of many other transitionmetal-containing materials. In the present work, we focus on interfaces between Ag(111) and CoPc or MnPc. The investigated TMPc’s (TM: Co, Mn) are planar molecules with D4h symmetry; the central metal atom is surrounded by the Pc ring (Figure 3, inset). Despite a large number of experimental and theoretical studies in the past decades, driven by the excellent perspective for various applications, essential details of the electronic structure and excitations of CoPc and MnPc are still unclear,13,17 22 in particular, the role of the TMPc metal atom in the interface formation. Our studies have been carried out by photoexcited electron spectroscopies such as X-ray photoemission (XPS), X-ray absorption (XAS, NEXAFS), and resonant photoemission spectroscopy (ResPES) with the intention to understand the interference of the 3d states of the central transition-metal atom in the absorption configuration as well as the electronic properties of the interface. XAS provides information about the nature r 2011 American Chemical Society
of the empty molecular orbitals of the absorbing atom. It monitors transitions from core level to unoccupied states (to the lowest unoccupied molecular orbitals, LUMO). Because of the resonant nature of the electron emission process near the XAS edge, with ResPES, it is possible to shed light on the nature of the spectral features in the valence band. Therefore, this technique is suitable to identify spectral features in the valence band and correlate them with specific chemical sites inside the molecule and to unoccupied states.23 ResPES has been successfully applied also in the case of metal phthalocyanines.23,24
’ EXPERIMENTAL METHODS The XAS and the resonant photoemission measurements were carried out at the third generation synchrotron radiation source BESSY II (Berlin) using the Optics-beamline and the endstation SurICat. The photon energies were calibrated comparing the binding energy (BE) of Au 4f7/2 peak excited by first- and second-order light. XAS were acquired in total-electron yield (TEY) mode, measuring the drain current, with a resolution of ∼100 meV at a photon energy of 400 eV. Detailed description for the experimental process can be found elsewhere.25 The Ag(111) substrate was cleaned prior to organic film deposition by repeated sputtering-annealing treatment, and the cleanliness was checked by XPS. Ultrathin films of CoPc and MnPc, purchased from Aldrich, were thermally evaporated on the substrate in ultrahigh vacuum (base pressure
